Vehicles that operate underwater are useful for performing tasks below the sea surface in such fields as deep water salvage, the underwater telecommunications industry, the offshore petroleum industry, offshore mining, and oceanographic research. (See, e.g., U.S. Pat. Nos. 3,099,316 and 4,502,407). Conventional unmanned subsurface vehicles can be broadly classified according to how they are controlled. Autonomous underwater vehicles (AUVs) are subsurface vehicles that are not physically connected to a support platform such as a land-based platform, an offshore platform, or a sea-going vessel. In comparison, remotely operated vehicle (ROVs) are those subsea vehicles that are physically connected to a support platform.
The typical physical connection between an ROV and a support platform is referred to as an "umbilical." The umbilical is usually an armored or unarmored cable containing an electrical and/or hydraulic conduit for providing power to an ROV and a data communications conduit for transmitting signals between an ROV and a support platform. An umbilical thus provides a means for remotely controlling an ROV during underwater operation.
ROVs are commonly equipped with on-board propulsion systems, navigation systems, communication systems, video systems, lights, and mechanical manipulators so that they can move to an underwater work site and perform a particular task. For example, after being lowered to a subsurface position, a remotely-located technician or pilot can utilize an ROV's on-board navigation and communications systems to "fly" the craft to a worksite. The technician or pilot can then operate the mechanical manipulators or other tools on the ROV to perform a particular job. In this manner, ROVs can used to perform relatively complex tasks including those involved in drill support, construction support, platform cleaning and inspection, subsurface cable burial and maintenance, deep water salvage, remote tool deployment, subsurface pipeline completion, subsurface pile suction, etc. Although they are quite flexible in that they can be adapted to perform a wide variety of tasks, ROVs are also fairly expensive to operate as they require a significant amount of support, including, for example, a pilot, technicians, and a surface support platform.
ROVs and other subsurface vehicles that are connected to a surface vessel by a physical linkage are subject to heave-induced damage. Heave is the up and down motion of an object produced by waves on the surface of a body of water. Underwater vehicles physically attached to a floating surface platform therefore move in accord with the surface platform. Therefore, when an underwater vehicle is located near a fixed object such as the sea bed, a pipeline, or a wellhead, heave-induced movement can damage both the vehicle and the fixed object. To alleviate this problem, devices such as heave-induced motion compensators and tether management systems have been employed to reduce the transfer of heave to underwater vehicles.
In contrast to ROVs, while underwater, AUVs are not subject to heave-mediated damage because they are not usually physically connected to a support platform. Like ROVs, AUVs are useful for performing a variety of underwater operations. Common AUVs are essentially unmanned submarines that contain an on-board power supply, propulsion system, and a pre-programmed control system. In a typical operation, after being placed in the water from a surface platform, an AUV will carry out a pre-programmed mission, then automatically surface for recovery. In this fashion, AUVs can perform subsurface tasks without requiring constant attention from a technician. AUVs are also substantially less expensive to operate than ROVs because they do not require an umbilical connection to an attached surface support platform.
AUVs, however, have practical limitations rendering them unsuitable for certain underwater operations. For example, power in an AUV typically comes from an on-board power supply such as a battery. Because this on-board power supply has a limited capacity, tasks requiring a substantial amount of power such as cutting and drilling are not practically performed by AUVs. In addition, the amount of time that an AUV can operate underwater is limited by its on-board power supply. Thus, AUVs must surface, be recovered, and be recharged between missions- a procedure which risks damage to the AUV and mandates the expense of a recovery vessel (e.g., a boat).
Another drawback of AUVs is that, without a physical link to a surface vessel, communication between an AUV and a remote operator (e.g., a technician) is limited. For example, AUVs conventionally employ an acoustic modem for communicating with a remote operator. Because such underwater acoustic communications do not convey data as rapidly or accurately as electrical wires or fiber optics, transfer of data encoding real time video signals or real time instructions from a remote operator is not efficient given current technology. As such, AUVs are often not able to perform unanticipated tasks or jobs requiring a great deal of operator input.
Other underwater vehicles having characteristics similar to AUVs and/or ROVs are known. These vehicles also suffer drawbacks such as subjection to heave, need for expensive support, poor suitability for some applications, lack of a continuous power supply, poor communications, poor capabilities, etc. Therefore, a need exists for a device to help overcome these limitations.